1. Field of the Invention
The present invention relates, generally, to refrigeration doors and, in particular, to an energy-free refrigeration door providing condensation control, thermal insulation, and a desired amount of visible transmittance. More particularly, the refrigeration door of the present invention achieves these desired characteristics through the application of a low-emissivity coating, without electrically heating the door. Throughout this application the term “refrigeration door” is meant to refer to a door used for freezers, refrigerators and similar units and cabinets. In addition, for purposes of this application the term “energy-free” (as in energy-free refrigeration door) means that electricity is not applied to the glass to heat the glass.
2. Discussion of the Background
Refrigeration doors for commercial freezers, refrigerators and the like are typically constructed of glass to allow the customer to view the products placed therein for sale without opening the door. However, when condensation forms on the glass (sometimes referred to as “fogging”), the customer is not able to see through the door to identify the products inside, which is undesirable from the standpoint of the customer and the store owner or retailer as well.
Moisture condenses on the outside of the glass refrigeration door because the surface temperature of the outside of the glass is reduced below the ambient temperature in the store by the colder refrigerated interior of the freezer or refrigerator. When the temperature of the surface of the glass drops below the dew point of the air in the store, moisture condenses on the surface of the glass. In addition, when a door is opened in a humid environment, the innermost sheet of glass, which forms the inside of the door, is also momentarily exposed to the ambient air of the store and condensation may form on the inside of the door as well. The condensation on the inside of the glass door also occurs because the temperature of the inside of the glass door is below the dew point of the ambient store air to which it is exposed.
As previously indicated, condensation on the glass door, which may become frost, prevents the customer from seeing the products for sale through the glass door. Consequently, when condensation or frost is on the glass door, the customer must perform the unpleasant task of opening the refrigeration door to identify the contents inside, which is impractical in a store with a large number of freezers or refrigerators. Not only is opening every refrigeration door tedious and time consuming from the customer's perspective, it is undesirable from the retailer's freezers and refrigerators, thereby resulting in higher energy costs to the retailer.
There are various industry performance standards which refrigeration doors are required to comply with in order to be acceptable. In the United States, much of the industry requires freezer doors (but not refrigerator doors) that prevent external condensation when used in an environment with an outside temperature of eighty degrees Fahrenheit (80 F), an outside relative humidity of sixty percent (60%), and an inside temperature of minus forty degrees Fahrenheit (−40 F). Other countries have different requirements.
As is well known in the art, a typical refrigeration door is comprised of an insulating glass unit (IGU) housed in a door frame. The IGU in a refrigeration door is, typically, comprised of two or three sheets of glass sealed at their peripheral edges by a sealant assembly, generally referred to as an edge seal. In an IGU comprised of three sheets fo glass, two insulating chambers as formed between the three sheets of glass. In an IGU comprised of two sheets of glass, a single insulating chamber is formed. Typically, IGUs for refrigerators are constructed of two sheets of glass, while IGUs for freezers employ three sheets of glass. Once sealed, the chambers are often filled with an inert gas such as argon, krypton, or other suitable gas to improve the thermal performance of the IGU.
Most conventional approaches to preventing or reducing condensation in a refrigeration door involve supplying energy to the door by including a conductive coating on one or more of the glass surfaces of the IGU for electrically heating the glass. The purpose of heating the glass is to maintain the temperature of the glass above the dew point of the warmer ambient air of the store. By heating the glass above the dew point, the undesirable condensation and frost are prevented from forming on the glass in the door, providing a clear view through the glass to the interior of the refrigeration compartment.
In a door consisting of a three-paned IGU, an unexposed surface of one or two of the sheets of glass is coated with a conductive material. The conductive coating is connected to a power supply by two bus bars or other electrical connectors mounted on opposite edges of the glass. As current passes through the coating, the coating heats, thereby heating the glass sheet to provide a condensation-free surface. The coating on the IGU of a refrigeration door is normally applied to the unexposed surface of the outermost glass sheet. However, because condensation sometimes forms on the inside of the inner sheet of glass, the unexposed surface of the innermost sheet of glass may also be coated for heating to prevent condensation.
There are numerous drawbacks and problems associated with these conventional heated refrigeration doors of the prior art. First, heating the door incurs an energy coast above and beyond the energy costs of the cooling system. In a standard size commercial freezer, the additional cost to heat a freezer door is substantial—based on current electrical utility pricing, such additional costs can be $100 per year or more for each freezer. Considering that many stores utilize multiple freezers, with some supermarkets and other food retailers utilizing hundreds of freezers, the cumulative energy costs associated with such heated freezer doors are significant.
Second, excess heat from conventional heated refrigeration doors will migrate to the refrigeration compartment, creating an additional burden on the cooling system, which results in still greater energy costs. Third, if the power suppled to the door for heating is too low, is turned off, or is shut down due to a power outage, condensation and/or frost will form on the glass. If the power dissipation is too high, unnecessary additional energy costs will be incurred. In order to reduce the occurrence of these problems, such heated glass doors often require precise control of the door heating system. In order to achieve the necessary precise control of the door heating system, an electrical control system is required, which results in increased design and manufacturing costs, as well as substantial operational and maintenance costs.
Fourth, these electrically heated glass doors present a safety hazard to customers and a potential risk of liability and exposure to retailers and refrigeration system manufacturers. The voltage applied to the glass door coating is typically 115 volts AC. The shopping carts used by customers in stores are heavy and metal. If the shopping cart strikes and breaks the glass door, electricity may be conducted through the car to the customer, which could cause serious injury or even death.
U.S. Pat. No. 5,852,284 and U.S. Pat. No. 6,148,563 disclose applying a voltage to a glass coated with a conductive coating (which may be a low emissivity coating) to control the formation of condensation on the outer surface of the glass door. The conductive coating, such as a low emissivity coating, provides a resistance to the electricity, which produces heat, while also providing desirable thermal characteristics. However, the refrigeration doors disclosed in these patents suffer from the previously described drawbacks and problems associated with all electrically heated refrigeration doors.
In addition to being used for conductivity, such low emissivity coatings have been employed as another means for reducing condensation on refrigeration doors. Specifically, one method of increasing the insulating value of glass (the “R value”), and reducing the loss of heat from the refrigeration compartment, is to apply a low emissivity (low E) coating to the glass. A low E coating is a microscopically thin, virtually invisible metal or metallic oxide layer(s) deposited on a glass surface to reduce the emissivity by suppressing radiative heat-flow through the glass. Emissivity is the ratio of radiation emitted by a black body or a surface and the theoretical radiation predicted by Poanck's law. The term emissivity is used to refer to emissivity values measured in the infrared range by the American Society for Testing and Materials (ASTM) standards. Emissivity is measured using radiometric measurements and reported as hemispherical emissivity and normal emissivity. The emissivity indicates the percentage of long infrared wavelength radiation emitted by the coating. A lower emissivity indicates that less heat will be transmitted through the glass. Consequently, the emissivity of a sheet of glass or of an IGU impacts the insulating value of the glass or IGU as well as the heat conductivity (the “U value”) of the glass or IGU. The U value of a sheet of glass or of an IGU is the inverse of its R value.
In a multi-pane IGU, the emissivity of the IGU, which is the combined emissivity of the sheets of the glass that form the IGU, may be approximated by multiplying the emissivity of all the sheets of glass together. For example, in a two-sheet IGU with each sheet of glass having an emissivity of 0.5, the total emissivity would be 0.5 multiplied by 0.5 or 0.25.
While low E coatings have been applied to IGUs used in refrigeration doors both with and without electrically heating the doors, such coatings and IGUs are not capable of controlling condensation and providing the required thermal insulation through the broad range of temperatures and environments in which such refrigeration doors are utilized without applying electricity to heat the doors. More specifically, notwithstanding the use of such low E coatings, refrigeration doors that are not heated have failed to provide condensation control in applications in which the interior temperature of the refrigeration compartment is substantially near or below freezing.
Thus, notwithstanding the available electrically heated and low emissivity coated refrigeration doors, there is a need for a refrigeration door: (1) that provides the necessary condensation control and thermal insulation over a broad range of temperatures and environments; (2) with the desired amount of visible transmittance; (3) that avoids unnecessary energy costs and undue burden on the cooling system by eliminating the need for supplying electrical power to heat the door; (4) that does not require an expensive and complex electrical control system, thereby minimizing design, manufacturing, operation, and maintenance costs; and (5) that does not present a safety hazard to customers and a potential risk of liability and exposure to manufacturers and retailers.